The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit and including dolomite with layers containing an organic polymer called "kerogen", which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing the oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; and 4,043,598, which are incorporated herein by this reference. Such patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale by fragmenting such formation to form a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen-containing retort inlet mixture into the retort as an oxygen-supplying gaseous combustion zone feed to advance the combustion zone through the retort. In the combustion zone, oxygen in the combustion zone feed is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. Temperatures are attained in the combustion zone sufficiently high to decompose carbonates of alkaline earth metals in oil shale to the corresponding oxides of alkaline earth metals. By the continued introduction of the retort inlet mixture into the retort, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the combustion zone feed that does not take part in the combustion process pass through the fragmented mass in the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products, including gaseous and liquid hydrocarbon products, and to a residual solid carbonaceous material.
The liquid products and gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, are collected at the bottom of the retort. An off gas containing combustion gas, including carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process, is also withdrawn from the bottom of the retort. The products of retorting are referred to herein as liquid and gaseous products.
Residual carbonaceous material in the retorted oil shale can be used as fuel for advancing the combustion zone through the retorted oil shale. When the residual carbonaceous material is heated to its spontaneous ignition temperature, it reacts with oxygen. As the residual carbonaceous material becomes depleted in the combustion process, the oxygen penetrates farther into the oil shale retort where it combines with remaining unoxidized residual carbonaceous material, thereby causing the combustion zone to advance through the fragmented oil shale.
At the end of retorting, a retort can contain a substantial quantity of oxides of alkaline earth metals formed by decomposition of carbonates of alkaline earth metals in oil shale. Concern has been expressed that these oxides of alkaline earth metals, as well as other water-soluble constituents of combusted oil shale, can be leached by ground water, thereby contaminating ground water. A high level of contaminants in ground water can make it unsuitable for some uses, including domestic uses.